Isan capital ships are all modular, held together with powerful couplings that allow an Isan fleet to build the optimal configuration of ships before a battle to execute its plan. Isan ships consist of one (or occasionally more) command modules (the D-Type, I-Type, L-Type, and T-Type) and at least several utility modules (the O-Type and X-Type). Command modules house admirals and their staff, a Nephelium drive, and a sub-light engine capable of driving the entire ship at combat speeds. The D-Type is the workhorse of the Isan Free Navy, the most basic and common command module, with a powerful engine that makes it a key building block for ships both by itself and in combination with other command modules. The D-Type is shaped like the head of a screw, with the exhaust ports on the narrower side: When an Isan ship is viewed in horizontal profile with the D-Type at the stern, it looks like a D.

The standard D-Type has a screwhead shape, about 100 yards in diameter at the widest point and tapering in toward the engine exhaust port at the rear. Isan ships are organized into a different style of deck than the Malleon or Feron: In Isan ships, gravity points toward the outer hull of the ship, and each deck is a concentric ring around the ship’s central axis, with “up” leading toward the central axis and “down” pointing at the outer hull. (Picture the rings from a tree stump.) This is powered by Nephelium-based artificial gravity, but it works like real-life artificial gravity where a spaceship spins to generate a centrifugal force. Early Isan aerospace projects often had to deal with Nephelium storms from Karsa that would temporarily shut down artificial gravity, and Isan engineers built their ships so that they could use centrifugal artificial gravity as a backup. The Navy stuck with this design, which proved crucial in the War because Isan ships could turn off their Nephelium drives entirely to hide their Nephelium signature while still being able to walk rather than float. Standard D-Types have a single coupling at the center of ship opposite the engine.

Most D-Types will stack several of the heavy O-Types and light X-Types using these couplings—the O- and X-Types have two sets of couplings so that they’re stackable—with the specific number and type defining the purpose of the ship. A frigate, for example, would have a small stack consisting mostly of X-Types, where a cruiser would stack more modules and consist mostly of O-Types. When modules connect using these power couplings, they form a rail that allows the entire ship to link up in a data network and share both conventional and Nephelium power wherever it’s needed. This is especially important because O- and X-Types don’t have their own Nephelium drives, so any tech that requires Nephelium energy only works when connected by the rail to a command module. At 100 yards, the D-Type has a longer diameter than the largest O-Type variants, which measure up to 80. This remaining ring allows for the mounting of turrets, hangar bays, and Nephelium torpedo tubes, with these also being detachable modules that recess into bays in the D-Type to allow for swappable configurations. Opposite the power coupling is the engine exhaust section, which consists of a huge number of very small exhaust ports compared to those on Malleon and Feron ships: Collectively they’re just as powerful as one big exhaust port, but the mini-exhaust design gives Isan engineers more minute control over speed and power distribution.

There are several variants of the D-Type, which fall into two categories: Larger versions of the D-Type and versions with couplings at different angles. In the first category, the DS-2-Type, for example, has a wider diameter than the D-Type and two off-center couplings to allow for two separate rails of O- and X-Types. Some DS-3-Type and DS-4-Types, with three and four couplings respectively, were also built in an effort to make Isan battleships to replace the many lost Malleon ones at the low point of the War. In the second category, the DV-Type is angled at the front rather than flat, to support two rails angled in a V-shape. Some of the rarer DY-Type, with three angled rails, were also built, and Isan engineers experimented with still more unusual shapes.

The Isan’s naval doctrine is called histagal, the idea that a battle can be won before the first shot is fired if the Isan can configure and tune their ships into the perfect force to exploit an enemy fleet’s weak points and counter its strengths. The D-Type is histagal at its most distilled: It’s mostly just a big engine to jet around its stack of O- and X-Type weapon/utility modules. A D-Type can function as everything from a frigate, with just several X-Types, all the way up to a heavy cruiser, with stacks of dozens of O-Types. It’s the least showy of all the Isan command modules, but the humble D-Type became synonymous with Isan adaptability and ingenuity.